This invention relates to transformers, a core for a transformer and an ignition system for a vehicle comprising a transformer.
A known vehicle ignition system transformer comprises a unitary solid or laminated core, such as a pencil core, of a magnetic material. Primary and secondary windings of the transformer are wound around the core. The transformer must comply with a number of requirements. The solid core must provide good magnetic coupling between the primary and secondary windings, so that energy can be transferred from the primary winding to the secondary winding during a single pulse. The primary and secondary inductances must be large enough so that sufficient energy can be stored in the magnetic core, so that the maximum primary current is not too high and so that the spark duration is long enough for a stable spark. The large secondary inductance requires a large number of turns. This results in the secondary winding having a resistance of several kilo-ohm. The resistance results in heating of the windings, which must be taken away. Hence, the transformer must provide for sufficient heat transfer from the windings to the outside of the transformer. The magnetic design must be such as to prevent core saturation during high voltage generation. Furthermore, enough magnetic material is required to store sufficient energy in the magnetic field. Very good electrical isolation is required between the secondary windings and the magnetic core. The maximum secondary voltage is normally larger than 30 kV and the magnetic core is normally conductive. The isolation between the core and windings must be able to withstand the maximum voltage. Sufficient isolation between the windings is also required. Because most magnetic materials meeting these requirements are conductive or have a low dielectric strength, a relatively thick isolation layer is required between the core and the secondary winding, which is undesirable. A transformer suitable for use in an automobile engine must be able to operate at temperature between about −40° C. and about +140° C. Due to different thermal expansion coefficients between the core and the isolation material, mechanical stresses develop. After a number of thermal cycles, gaps or cracks between the magnetic material and isolation material may develop, which may be fatal.
To achieve these requirements while also reducing the volume of the transformer becomes very difficult. Because of the large number of turns in a small volume, the capacitance of the winding (including inter-turn capacitance) becomes large, which results in more energy required to generate a certain high voltage.